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Nancy Overton, who joined the pop group the Chordettes in 1958 and melded her mellifluous low tones into their barbershop-quartet-like harmonies on some of their later hits, died on Sunday in Blairstown, N.J. She was 83 and lived in Hackettstown, N.J.

The cause was esophageal cancer, her son Rick said.

Ms. Overton sang on the original recordings of two Chordettes hits: “Zorro,” a vocal version of the theme from the hit television show that climbed to No. 17 on the charts in 1958, and “Never on Sunday,” the title song of the movie starring Melina Mercouri, which made it to No. 13 in 1961.

The Chordettes had been together for 10 years when Ms. Overton stepped in for one of the original four, Janet Ertel, who no longer wanted to go on the road. With singles like “Eddie My Love” in 1956 and, particularly, “Mr. Sandman” in 1954, the Chordettes had already made the bobby-soxer generation swoon.

Before the Chordettes, Ms. Overton had sung with the Heathertones, making recordings with the likes of Benny Goodman and Tommy Dorsey.

The Chordettes had already recorded what would become their second-biggest hit, “Lollipop,” before she joined the group. With its bubble-gum lyrics — “Lollipop, lollipop, oh lolli, lolli, lolli …” — the song reached No. 2 on the charts. As the group reprised the song on television variety shows, Ms. Overton sang the bass line, punctuating that repetitive line with a pursed-lip pop.

Shortly after the release of “Never on Sunday,” the group disbanded.

Anne Swain (she preferred her childhood nickname, Nancy) was born in Port Washington, N.Y., on Feb. 6, 1926, one of two daughters of Philip and Edith Swain. Her father was a physicist; her mother played the organ in church and taught singing.

She married Hall Overton, a jazz pianist, in 1946; he died in 1972. Besides her son Rick, of Los Angeles, Ms. Overton is survived by another son, Steven, of Placitas, N.M.